Since an aminoplast obtained by the reaction of a compound having amino group, acid amide bonding or the like, and formaldehyde, as, for example, melamine resin, urea resin, aniline-formaldehyde resin and the like, has such active group as active hydrogen, active methylol and active alkoxymethylene, it is generally combined with various resins for coating use having a functional group which is reactive with the abovesaid active group, as hydroxyl group, isocyanate group or the like, including acrylic resin, alkyd resin, polyester resin, epoxy resin, polyurethane resin, polyamide resin, polycarbonate resin and the like, and used as heat curing type coating composition.
Various isocyanate compounds are also used, with said base resins, in heat curing type coating compositions. However, such coating compositions generally require high temperature baking, and in the combination of said resins and aminoplasts, are practiced the baking at about 140.degree. C. for the combination of alkyd resin and melamine resin for automobile use and the baking at about 170.degree. C. and more for the combination of epoxy resin and amino resin for coil coating use.
From the stanview of energy saving, it is, of course, desired to have a lower baking temperature and for this end various proposals have been made. One of the proposals heretofore made is to increase an acid value and hydroxyl number of such resin as alkyd resin, thereby improving the reactivity with aminoplast, and to increase the molecular weight of the resin, thereby improving the curing property thereof. However, this inevitably accompanys with such problems as undesired increase in paint viscosity, lowering of storage stability and decrease in water resistance of the formed film. Attempts have also been made to increase the molecular weight of aminoplast and improve the curing property of the coating composition but the results have been proved to be null because of causing additional problem of poor compatibility with alkyd and other resins.
There is another proposal of adding an external catalyst as p-toluene sulfonic acid, phosphoric acid and the like to the combination of coating use resin and crosslinking agent. However, no satisfactory results are obtained because of the poor pigment dispersion stability, e.g. segregation, due to the presence of external acid catalyst, poor storage stability due to the gradual curing of the coating composition at room temperature and lowering of water resistance of the film. In case that the isocyanate compounds are used as crosslinking agents, employment of external catalyst as tin compound, amino compound and the like, will cause decrease in pot-life of the coating composition, resulting the inferior working properties thereof.
From the foregoing, the inventors, having made endeavor for obtaining a coating composition which comprises a coating use resin and crosslinking agent, has an improved curing property without the necessity of using an external catalyst, is curable at a lower temperature or in a shorter period of time, and is excellent in storage stability, capable of resulting excellent film performance, have completed the invention.